The present invention relates to signal control systems in general and in particular to a signal control system for controlling the amplitude of signals in a multi-microphone sound reinforcement system.
The dream of an effective voice-operated reinforcement system (VOX) has been pursued by creative sound men ever since the problem was recognized. The stumbling block on which most attempts have failed is the method by which the need for a microphone to be on is detected. Excellent performance has been obtained in systems in which the detection was by means other than acoustical, such as foot mats or push-to-talk switches. Gain control to prevent feedback in these systems has been achieved either through logic which prevents more than a small number of mikes from being open at the same time, a master gain adjustor responsive to the number of open microphones, or a combination of these methods. The general disadvantage these systems share is in the amount of special equipment which must be custom-built for each job.
Voice-operated systems are frequently tried but rarely effective in increasing the gain of a sound system. This is due to the inadequancy of the process used to detect the active microphones, gating with a fixed threshold. VOX devices can be effective in reducing ambient noise pickup in a multi-mike system as long as the noise level is always less than the minimum speech level. This condition does not exist in most meeting rooms. It is impossible to find a satisfactory setting for VOX controls in most applications. If the VOX threshold of a mike channel is set at a low enough level to detect the minimum level signals that occur, such as a weak talker who is a bit off-mike, then any loud sound in the room, such as a strong talker at another mike, music, or applause will turn on the mike. If, to prevent this false triggering, the VOX threshold of an input is set at a high enough level to reject noise and the loudspeaker's sounds, then it will be too insensitive to pick up soft speech. The audible effect will be irritating, intermittent speech under marginal conditions.
The performance of VOX devices can be optimized by making their control circuits frequency discriminatory. The gate can be made to trigger more readily on signals with a typical speech spectrum, which makes the unit less sensitive to ambient noise. This type of improvement is not sufficient to give increased gain in a sound reinforcement system. It is always possible for some sound to turn on a number of mikes or all mikes. The resulting feedback howl will keep all the VOX's open.
Experimental and practical systems have been built in which either VOX or manual gating of the input channels is used, and an automatic attenuator reduces the master gain in accordance with the number of inputs activated at any time. Such an automatic control function looks like this:
______________________________________ NOM Master gain adjustment dB ______________________________________ 1 0 2 -3 3 -4.8 4 -6.0 5 -7.0 * * 10 -10.0 * * 20 -13.0 * * 100 -20.0 ______________________________________
where NOM is the Number of Open Microphones. A system with such an automatic attenuator will provide the maximum possible gain for the number of mikes on at any time. The runaway feedback condition, in which the feedback keeps the VOX's open, will not occur. If an automatic master gain control of this type were combined with a voice-operated gate which worked well, a fully automatic mixing system could be created.
The solution to th problem of accurately detecting when a microphone chanel should be gated on lies in the use of an adaptive threshold for the input chanel gates. If the thresholds of the VOX gates are continuously adjusted to follow the ambient sould level conditions, then individual channels will open only when their signals exceed the ambient sound level. The criterion for a mike being on becomes the signal-to-noise ratio at the mike, rather than the sould level. This little-known technique has been applied to special communication systems but, so far as is known, has not heretofore been applied to sound reinforcement. Unfortunately, the conventional VOX gate cannot tell the difference between sounds which originate near the mike and far away; it acts only on the level of the signal.
If a source of sound is beyond the critical distance from the microphone or below the ambient noise level, it is not possible to amplify it in a reinforcement system. Experienced mixers know that if a microphone's gain is raised trying to reach for a source of sound which is too far away or lost in the noise, the result will be feedback or mush. Conversely, the sound system is better off for both noise and gain whenever mikes which aren't picking up anything useful are turned down. The adaptive threshold gating technique makes its decision on just the right criterion: is the mike hearing anything special besides the general sound in the room?
Experiments in the field have confirmed that the adaptive threshold gate is an extraordinarily accurate and reliable indicator of the need for the microphone to be on in both speech and music systems. A system embodying this principle was prototyped, and is described in applicant's U.S. Pat. No. 3,814,856.
As described in U.S. Pat. No. 3,814,856, the most straightforward way to obtain an ambient noise threshold is to monitor the sound level in the area with a microphone. In the patent there is described a mixing system applying the combination of adaptive threshold gating with number-of-open-microphones gain adjustment. Only a limited number of input channels are shown. The signal path to the output is identical to the circuit of a conventional mixer except for the introduction of voltage-controlled gain elements in each input channel and in the master mixed signal channel. Since the range of gain shift necessary is not great (around 20 dB maximum) these may be simple photoconductive elements which contribute no measurable noise or distortion.
Outside the audio signal path an envelope detector circuit produces a DC control voltage representing the audio level in the chanel, which is anaogous to the sound level at the microphone. An identical envelope detector monitors the noise microphone channel. A comparator associated with each input channel observes the ratio of signal to noise in that channel and sends the "go" command to the attenuator-gate whenever it is satisfactory. The acoustic S/N ratio at which each channel turns on its determined by the setting of the threshold control which biases the comparator. The optimum threshold setting is that which causes the channel to turn on just below the input level which would make any perceptible contribution to the resulting mix. Thus the turning-on of the gate is masked. Thinking of this in terms of mike working distance, if the threshold is set to turn the mike on from a source which is just a bit too far away to get any useful gain, then the channel will always be on when the source is audible, and conversely switching will not be audible.
When the ambient sound level is low, the gates become extremely sensitive, and will open for a whisper several feet from the mike. One can imagine a "balloon" os sensitivity extending out from the mike to the threshold distance where the noise level equals the source. When the ambience is high level, such as the case with vocal mikes in front of an electric band, the threshold becomes correspondingly high and the balloon contacts to less than an inch. This also corresponds to the useful working distance of the mike under those conditions.
It is necessary for the envelope detector circuits to be able to handle a wide dynamic range, even in speech systems. The sound levels which may appear at a microphone range from 30 dB in a quiet room to over 120 dB from a close-talking loud voice. In practice an 80 dB range has been adequate.
The master gain adjustor circuit monitors the number of open microphones by summing the control voltages of all the inputs, and using this voltage to actuate an attenuator in the master channel which is made to have the proper curve for 3 dB gain reduction whenever the NOM is doubled.
The present improvement is the result of a combination of improvements in the original invention. Some details of this development process will shed more light on the principles of both the original invention and the improvement.
Channel attenuators began by being very fast on-off gates and subsequently went through many changes of dynamic and attenuation characteristics. The optimum attenuation when a channel is in the off condition is just enough to keep the channel out of trouble. Rather than turning the input channel on and off, the gates need to shift it between two levels, "hot" and "safe." This is the technique that an experienced human mixer uses. Mikes which are standing by are kept turned down but not off, so that they don't have so far to go when they come up. It turns out that the optimum amount of attenuation for the input channel gates is that which, combined with the effect of the master gain adjustor, makes the "all mikes off" total system gain identical with the "all mikes on" total gain. This at first seems to be a paradox, but it actually is a key to smooth operation. Ideally, the amount of attenuation of the off channels should vary depending on how many channels are on, but it was found that a fixed amount of attenuation could produce very satisfactory performance. The amount of attenuation was set to be the power ratio of the gain of one mike to the gain of six mikes (the prototype had six inputs) plus 3 dB. The table shows the results of the interaction of the individual channel attenuators with the master gain adjustor.
__________________________________________________________________________ master total input gains dB gain system CONDITION 1 2 3 4 5 6 adj. dB gain dB __________________________________________________________________________ all off -11 -11 -11 -11 -11 -11 +3 0 one mike in use 0 -11 -11 -11 -11 -11 0 +0.4 2 mikes in use 0 0 -11 -11 -11 -11 -3 +0.5 all mikes in use 0 0 0 0 0 0 -8 0 __________________________________________________________________________
Note that the total gain of the system remained substantially constant under all conditions of use.
There are two advantages to configuring the system this way. First, the threshold of feedback remains the same no matter how many mikes are in use. This means the system automatically mixes for maximum acoustic gain under varying conditions of use. Second, the pickup of ambient noise by the system remains substantially constant, which is very important in recording and brodcasting. "Pumping" or "breathing" of background noise is the most-often heard flaw in automatic audio gain control systems. The level of ambient noise picked up by the system will alway be about the same as is picked up by one mike at normal gain, no matter how many mikes are actually open. This system, then, not only approaches "one mike" gain in multi-mike installations, it also approximates one-mike noise pickup.
Since the gain shifts in the system are quite small, they are masked by the much larger shifts in signal level which stimulate them. For example, the largest master gain shift is 3 dB, the difference between one and two mikes operating. When an instrument or voice on one mike is joined by a second on another, perception of the 3 dB drop in level which is imposed upon the first signal is masked by the perception of the entrance of the second sound. These shifts are really small compared to the large amounts of limiting and compression used in popular music recording. They are not perceptible in test recordings made from the mixer prototype during live musical performances.
In speech systems, it was discovered that a much more accurate reference signal could be obtained from the sum of all the inputs (before the attenuators) than from a separate reference microphone. What better way could there be to sample the sound level in the area of the microphones than with the microphones themselves? The multi-mike sample is much more accurate due to the averaging of standing waves.
The pursuit of errors in system operation always led to significant improvements in performance One error which led to a very significant step was the "sailing" problem. The input channel attenuator at the time had a moderately fast attack, 20 milliseconds or so, fast enough to catch attack transients of consonants but not so fast as to cause a transient of its own when interrupting low frequency sounds. The decay was slow, 400 ms or so. The error occurred when a singer sang the word sailing softly while instruments sustained a note. It would come out "tailing" because the input gate would not trigger until the level of the s sound had come up out of the ambient level, and by that time it was too late. There was no way to deal with this in a simple "go-no go" gating system. Two alternatives were considered: going to a multi-band (like Dolby) system which would improve masking, or making the attack of the signal control the speed of the attack of the gate. The latter was chosen.
The comparator and gate circuits were reconfigured to have a 10 dB wide "expansion window" instead of a point threshold which simply caused the gate to trigger on. When the input signal rises above the lower threshold of the window, instead of triggering up to high gain it is expanded with a 2/1 slope until it reaches its normal on gain, and then acts linearly as long as the input stays above this upper threshold. In this way the rate of gain increase is controlled by the rate of signal increase, and the same gain shifts can be accomplished as before, but much more smoothly and without the sailing error.
This system can be packaged as a conventional mixer or included as a feature of a console. A threshold control is required for each input, with an indicator showing the condition of the gate.
During tests and demonstrations of the system described above, an apparent malfunction was observed Under certain conditions, when two microphones were actuated at the same time, each of the two channels would come up only to 3 dB below normal gain, and the master gain adjustor, which was being monitored with a meter, would stay at 0. The total gain would be correct, but the 2 dB attenuation that was supposed to be coming from the master channel was coming from the individual channels instead. This anomaly was reproduced in the lab and it was found that it occurred in the mode of operation where the sum of the inputs was furnishing the reference, and the thresholds were set so that an input to one channel only would cause that channel's gain to ride just at the top of the 2/1 expansion window.
Analysis of the system voltages when the thresholds were set at that point revealed that a new control function had been discovered, making possible a much simpler system than had ever been imagined.